The present invention relates to an improved aspheric mirror, and method of designing the same, for use in a lithographic system adapted to expose a workpiece, such as a semiconductor wafer, to a specified type of radiation, e.g., X-rays. More particularly, the invention relates to a mirror design that delivers uniform illumination over a narrow straight line suitable for use in a lithography system or as an input to a monochromator.
X-ray lithography is used as a means of processing solid state devices, such as semiconductor wafers. For example, lithography processes are used to form desired circuit and device patterns on the wafer as a particular semiconductor device is made. The rapidity and sharpness with which desired circuit and device patterns can be made using X-ray lithography is a function of the intensity and uniformity of the X-ray exposure of the wafer. Hence, as the demand for smaller and more dense semiconductor devices has increased, there has thus been a corresponding need for more intense and uniform exposure of the semiconductor wafer.
For most purposes, an X-ray source may be considered as a point source, with its radiation emitting in a ray fan from the point of origin. Hence, in using such radiation, a balance must be reached between the desire to use as much of the emitted radiation as possible, in order to minimize exposure times, and the desire to limit the variation of incidence angle across the exposed workpiece, which variation of incidence angle may cause undesirable variations in the power density of the X-ray. The angular limitation is typically accomplished by placement of the workpiece far from the X-ray source. This however is at the expense of reduced intensity and longer exposure time. What is needed, therefore, is a way to place the workpiece relatively close to the X-ray source, while capturing as much of the radiation as possible to uniformly expose the workpiece.
One way known in the art to direct as much of the X-ray radiation as possible towards the workpiece is through collimation. Collimation may be achieved, for example, through the use of a mirror, as shown, e.g., in FIG. 4 of U.S. Pat. No. 4,028,547. Unfortunately, while such mirror provides collimation of the X-ray beam in one direction, and thereby directs most of the beam to the workpiece, it does not necessarily uniformly expose the workpiece with the beam directed thereto. That is, the power distribution or intensity of the radiation within the collimated beam may vary through a cross section of the beam, thereby exposing the workpiece nonuniformly. Hence, what is needed is a mirror that will not only provide the needed collimation of the beam in one direction, such as the horizontal direction, but that will also uniformly focus the beam in another direction, such as the vertical direction.
Even if a mirror provides the requisite beam collimation in one direction and beam focussing in the other direction, however, thereby creating a substantially straight focal line of radiation on the workpiece, such radiation will not produce the desired uniform exposure of the areas of the workpiece it covers unless the straight focal line contains a substantially uniform linear power distribution. Hence, there is a need for a mirror that provides a straight focal line having a uniform linear power distribution. Further, if the straight focal line is scanned or swept across the workpiece to uniformly expose the entire surface area of the workpiece to the desired radiation, there is also a requirement that such scanning not compromise the straightness or uniformity of illumination of the focal line, i.e., the focal line must remain substantially straight and maintain its uniform linear power distribution at all locations of the scan or sweep.
A toroidal mirror with its two radii appropriately chosen may be used to inexpensively provide collimation in the horizontal direction and focusing in the vertical direction of an incident X-ray beam from a point source. Unfortunately, however, the focal line of the radiated energy on the workpiece from such a toroidal mirror is neither straight nor of uniform power distribution. Rather, even for a perfectly made toroidal mirror, the focal line is curved and spreads, and the curvature changes, as the line is scanned across the workpiece, thereby causing numerous aberrations that make the uniform and efficient exposure of the workpiece virtually impossible. Use of a toroidal mirror also requires that a vacuum window, used at the end of the beamline, either be designed to be large enough to accept the whole aberrated image (and thus be thick and lossy), or be designed smaller (in which case it can be thinner) but with more losses incurred on the window frame which would be non-uniform with respect to both position along the line and across the scan. What is clearly needed, therefore, is a new type of mirror that has been shaped and polished in a prescribed manner so as to correct for, eliminate, or minimize any optical aberrations that tend to curve or spread the focal line, both as it is imaged on the workpiece, and as it is scanned across the workpiece.